Hyperpolarization-activated cyclic nucleotide gated cation channels (HCNs) generate the Ih current. In the hippocampus, Ih regulates neuronal resting membrane potential and control excitation in the hippocampal circuit. Of the four HCN genes, HCN1 and HCN2 are prominently expressed in the hippocampus, with HCN1 as the dominant HCN isoform in CA1 pyramidal neurons. We have shown that hyperthermia-induced seizures, in the immature rat hippocampus, altered HCN expression by increasing HCN2 and reducing HCNI transcripts. The alteration on the relative expression HCN1 and HCN2 in the hippocampus may be responsible for the alterations on the functional properties of Ih, and the hyperexcitability of the hippocampus after early-life seizure events, e.g., febrile seizures. However, it is not known what molecular mechanisms are involved in the activity-dependent regulation of HCN genes. Since thyroid hormone mediates the regulation HCN channel in the heart, we predicted that thyroid hormone is required for the activity-dependent regulation of the HCN genes in the hippocampus. In this work we will induce seizure-like activity on organotypic hippocampal slice cultures in the presence and absence of thyroid hormone. Radioactive in situ hybridization and quantitative linear antisense mRNA amplification approaches will be used to quantify changes in HCN1 and HCN2 mRNA expression. These methodological approaches wilt help elucidate the molecular mechanisms involved in the seizure-induced alteration of HCN expression.